This invention relates generally to forming hollow plastic products and more particularly to an apparatus and method for handling a parison and transporting the parison to a mold.
Hollow plastic products such as some fuel tanks and similar products, have been made by an extrusion and blow molding process. In this process, a molten plastic parison is formed by extrusion and then transferred to a blow molding machine wherein a pressurized fluid is introduced into the interior of the parison within the mold to force the molten parison into is engagement with the interior walls of the mold. When cooled and solidified, the molded plastic part will have an exterior shape corresponding to the interior of the mold.
After extrusion and before being blow molded, the parison is hollow and has a generally cylindrical or tubular shape. In some applications, such as the production of automotive fuel tanks, the parison is large and may have a substantial length on the order of eight feet or longer. During transfer of the parison from the extruder to the blow molding machine it is imperative to prevent the collapse of the parison wall upon itself to maintain the hollow center of the parison in which the pressurized fluid is received to expand the parison within the mold.
Plastic products formed from prior blow molding processes have had non-uniform wall thicknesses with corners or other sharply contoured areas, and especially upper corners or contoured areas in the upper portion of the molded product, having substantially thinner walls compared to other portions of the molded product. Thus, to ensure that these contoured portions and especially the upper corner areas of a molded product have at least a necessary minimum wall thickness, the other portions of the product must be formed with walls which are thicker than necessary thereby substantially increasing the amount of material and the cost to manufacture the product.
One device used to transfer a molten parison from an extruder to a mold is disclosed in Canadian Patent No. 2,135,544. In this ""544 patent a parison carrying device has a collar which engages the parison after it emerges from the extruder and transfers the parison to a mold without pinching or collapsing of the parison walls. The collar has first and second halves which are made to grip the parison and which have fingers which are either fixed and hence, not movable, or which may pivot about a fixed axis to enable the collar to pinch and close one end of the parison to permit an increased pressure to be developed within the interior cavity of the parison. These fingers whether fixed or pivoted about a fixed axis, do not provide any stretching or redistribution of the parison material and therefore, plastic products formed with use of this parson carrying device have the aforementioned problem of non-uniform wall thickness with upper corners and other contoured portions of the product formed with thinner walls than other portions of the product.
A parison handling device has a pair of opposed arms with each arm having at least a pair of grippers to selectively engage an extruded parison and transfer it to a second location such as to a mold of a blow molding machine. At least one gripper on each arm is both slidably and generally pivotally movable relative to the arm to facilitate initially engaging and gripping the parison and subsequent closing and stretching of a gripped end of the parison to more efficiently distribute the material of the parison such that after blow molding of the parison, the molded product has a substantially uniform wall thickness throughout. Preferably, a follower attached to each movable gripper is responsive to a profile or contour of a cam carried by each arm to guide the movement of the gripper corresponding to the profile of the cam. By adjusting or changing the profile of the cams, the slidable and pivotal movement of the grippers relative to the arm can be controlled to control the stretching of the parison and hence the distribution of the parison material as desired to form a particular product. By forming a product with a more uniform wall thickness, substantially less material is needed to form the product and therefore, the product may be formed at significantly lower cost.
Each arm preferably has three grippers thereon with each of a pair of movable grippers disposed on either side of a middle gripper fixed to the arm. Each movable gripper is driven by an actuator between retracted and advanced positions to facilitate enaaging the molten parison and thereafter closing and stretching a gripped portion thereof. Desirably, when the arms are closed and the grippers are in their advanced positions a generally hexagonal geometry is defined to increase the surface area of engagement between the grippers and the parison.
Objects, features and advantages of this invention include providing a parison handling device which facilitates transfer of a molten parison to a mold, prevents undesirable collapsing of the parison before it is molded, stretches a molten parison before it is molded to more effectively distribute the material of the parison relative to the mold, enables a blow molded product having corner areas and other contoured portions to be formed with a substantially uniform wall thickness, substantially reduces the material needed to form a blow molded product, substantially reduces the cost to form a blow molded product, is readily adaptable to form products of different shapes and sizes, is reliable, durable, of relatively simple design and economical manufacture and assembly and has a long useful life in service.